1. Field of the Invention
The present invention relates to a wiring board.
2. Description of the Related Art
Conventionally, a so-called build-up wiring board is known in which wiring layers and insulating layers are alternately stacked, and the wiring layers are connected with each other through via holes that penetrate the insulating layers. In such a wiring board, the wiring layer is formed by forming a resist layer provided with open portions, depositing copper with a certain thickness in each of the open portions by electroplating or the like and removing the resist layer, for example (see Patent Document 1, for example).
However, in some of the wiring layers, a portion in which narrow wiring patterns are provided (portion whose line/space is narrow) and a portion in which wide wiring patterns are provided (portion whose line/space is wide) are mixed.
Further, the thickness of the resist pattern used for forming the wiring layer is the same at any portions of the wiring layer. This means that even though wiring patterns or various lines/spaces exist in one of the wiring layers, the thickness of the resist pattern is the same for all of the various wiring patterns regardless of the lines/spaces.
Thus, when the thickness of the resist pattern is the same for the narrow wiring patterns and the wide wiring patterns, an aspect ratio (a ratio of the thickness with respect to the width) of the resist pattern becomes relatively high at the narrow wiring patterns compared with that at the wide wiring patterns. Further, the resist pattern at the narrow wiring patterns is formed with a relatively narrow pitch.
The resist pattern is peeled by having peeling solution containing sodium hydroxide or the like absorbed by the resist pattern to be swollen. By having the resist pattern swollen, mechanical distortion due to volume expansion is generated in the resist pattern and the resist pattern is peeled.
Thus, compared with a portion of the resist pattern whose aspect ratio is low, it is difficult to remove a portion of the resist pattern whose aspect ratio is high. At the portion whose aspect ratio is high, there exists a resist pattern with a narrow pitch and high aspect ratio by being interposed between wiring patterns with a narrow pitch and high aspect ratio. Thus, at this portion, the volume of the resist pattern interposed between adjacent wiring patterns becomes relatively small, and the volume expansion amount in accordance with the absorption of the peeling solution and swelling is reduced.
Thus, the mechanical distortion amount of the resist pattern is reduced and defect in peeling tends to occur. Further, the dimension of the side surfaces of the resist pattern interposed between the adjacent wiring patterns are also increased, and defect in peeling further tends to occur. This tendency becomes significant as the resist pattern becomes to have a narrow pitch and high aspect ratio.